Now you are getting closer: Trains are good for transporting heavy goods. But they do have drawbacks when you want to ship light goods or need a very fast tact rate.
I could see a setup like this work to connect the world's two biggest harbors: Rotterdam & Antwerp. The distance would be about 70km (ca 50 miles). If it were set up to carry standard sea containers bidirectionally, this could effectively make one harbor what used to be two.
A truck today can carry one container one way in 90 minutes + loading/unloading - dependent on traffic. You don't want to make a container ship waiting on a truck because of traffic, and a container ship could get hundreds of containers from the other harbor.
Also, this infrastructure can be build with limited impact on the environment, and can go over land or low water with little difference.
Another benefit is the lower tact rate. If you need a container to get to the other harbor within the next 3 hours, its unlikely you can do that by train (trains don't go with just one container). With this system, you just "cut the line" to get there faster.
In short: Use rail/train to move the bulk goods that have time, and use this new system to move containers in under 60 minutes.